1. Field of the Invention
The present invention relates to a switching regulator which outputs a constant voltage, and more specifically to an overcurrent protection circuit which protects the circuit by suppressing a supply of a current to an output terminal when an overcurrent flows to the output terminal.
2. Background Art
A switching regulator is used as a voltage source for circuits of various electronic devices. The switching regulator is able to output a constant voltage to an output terminal regardless of voltage variation of an input terminal, but when the current that is supplied to a load from the output terminal rapidly increases thereby exceeding a maximum allowable current, it is important for an overcurrent protection circuit to protect the circuit by suppressing a supply of the current.
FIG. 4 is a block diagram of a switching regulator control circuit of the related art.
The switching regulator control circuit of the related art is configured to have a triangular wave oscillation circuit 1, an error amplification circuit 2, a PWM comparator 3, an error amplifier output detection circuit 4, a timer circuit 5, an AND circuit 6, a reference voltage circuit 7, and a buffer circuit 8.
The reference voltage circuit 7 outputs a reference voltage Vref1, and the triangular wave oscillation circuit 1 outputs a triangular wave Vramp which oscillates between an upper level VH and a lower level VL. The error amplification circuit 2 compares a feedback voltage Vfb of an output voltage Vout of the switching regulator with a reference voltage Vref1, amplifies a difference voltage therebetween, and outputs a voltage Verr. The PWM comparator 3 compares the voltage Verr of the error amplification circuit 2 with the triangular wave Vramp, and outputs a PWM signal Vpwm. The AND circuit 6 performs a control based on outputs of the timer circuit 5 and the PWM signal Vpwm. The buffer circuit 8 performs a power amplification of an output of the AND circuit 6, and outputs the amplified signal to a driver transistor (not illustrated). The error amplifier output detection circuit 4 monitors the voltage Verr of the error amplification circuit 2. The timer circuit 5 starts a count according to the output result of the error amplifier output detection circuit 4.
Here, the error amplifier output detection circuit 4 includes a reference voltage circuit which outputs a reference voltage Vref2 (>VH), and a comparator which compares the voltage Verr of the error amplification circuit 2 with the reference voltage Vref2. At the time of an overload state which is Verr>Vref2, the comparator outputs an overload state detection signal to the timer circuit 5. The timer circuit 5 starts the count, and outputs the overload state detection signal to the AND circuit 6 after a predetermined time elapses. Then, the AND circuit 6 controls the driver transistor so as to be turned off. Since the output voltage Vout decreases until 0 V, a difference voltage between the reference voltage Vref and the feedback voltage Vfb increases, a relationship of Verr>Vref2 is maintained, and the driver transistor is kept turned off. Thus, the output transistor is protected from the overload state (for example, patent document 1).
[Patent Document 1] Japanese Patent Application Laid-Open No. 3-52556